1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotating force of a motor, and more particularly to a device for controlling the operation of such a power steering system.
2. Description of the Prior Art
In the past, a conventional power steering system of this type, as schematically illustrated in FIG. 7, has hitherto been known in which a steering wheel 1 is operatively connected through a steering shaft 2 and a first rack and pinion gear T.sub.1 including a first pinion 5 and a first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a and 8b so that when the steering wheel 1 is turned by an operator, the steerable road wheels (not shown) are appropriately steered in accordance with the steering motion of the steering wheel 1 imparted by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear T.sub.2 including a second rack tooth portion 6b and a second pinion 18 and a speed-reduction gear R so that the driving force of the motor 13 is transmitted through the speed-reduction gear R and the second rack and pinion gear T.sub.2 to the rack 6 so as to assist the steering operation of the steering wheel 1 imparted by the operator. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9. The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operation of the motor 13 and the switching clutch 17 on the basis of the steering torque and the vehicle speed measured. The steering torque-sensor 3 is mounted on the steering shaft 2 for measuring the operator induced steering torque resulting from the steering force exerted on the steering wheel 1 by the operator, and has a sensor element 3a (FIG. 8) which is connected via a connector means or slip rings 19 to the control unit 9 so that the measured steering torque of the steering wheel 1 is converted by the torque sensor 3 into a voltage signal which is in turn transmitted to the control unit 9 by way of the connector or slip rings 19.
With the conventional power steering system as constructed above, the torque sensor 3 only serves to convert the measured steering torque into a voltage signal and no provision is made for preventing a voltage drop at the connector or slip rings 19 and/or a conducting line 20 connecting between the torque sensor 3 and the control unit 9. Such a voltage drop during transmission of the voltage signal necessarily results in a change in the measured steering torque value to be input into the control unit 9, thus making it possible for the control unit 9 to precisely control the power assisting operation of the motor 13 in accordance with the measured steering torque value.